The HD600 don't present a lot of the nasties and they don't have any real peaks/valleys in their response. They sound like a nice big smooth loudspeaker. Only real problem is the bass, where every recording sounds the same in that area, a bit of a puffy, tubby quality around 100Hz. The velvet cushion covers absorb a lot of upper midrange and treble (maybe excess m&t) but the sound is very smooth and relaxing and very polite and well mannered, even boring at times. Sort of like a very honest, intelligent, educated person who puts you to sleep after a while. But this comes with the benefit of never sounding harsh, grating or strident. It's sound is more analytical than musical, sort of pointing out the ingredients of the recording at the expence of the musical message. They are very smooth and uncolored in the midrange. Great for recording.
